This invention relates to magnetic-disc data storage devices having movable read-write heads.
In such devices data is arranged on the discs in tracks, and a single head is movable to access a number of tracks. During a transfer the head is positioned over a track and should be aligned with it accurately. This ensures, if the head is reproducing, that it receives a signal of the maximum strength free from interference from neighbouring tracks, and, if the head is recording, that the neighbouring tracks are not overwritten and the data is positioned in the expected place for a subsequent read operation. The more accurately the head can be positioned, the closer the tracks can be packed to one another on the disc.
When the storage device uses multi-disc packs with a head for each surface and the different heads coupled together it has been proposed to devote one surface to servo tracks which the head for that surface can be caused to follow. The remaining heads can thus be brought to reproducible positions on the discs. But this arrangement is wasteful of space that could be used to carry useful data, and is impractical for disc packs with only a few data surfaces.
An alternative is to position the head by reference to a separate position transducing system mounted in the device itself.
The head will then be fixed relative to the body of the storage device. However, if the track is not an accurate circle centred on the axis of rotation of the disc its radial position may vary, and the track will not remain accurately beneath the head. Such variations may occur if the disc is exchangeable and has been recorded on a different drive unit. The track, though circular, may then be off-centre when mounted on a unit other than the one which it was recorded. A similar eccentricity may occur even on the unit on which the disc was recorded if it has been taken off and replaced in a different angular position.